1. Field of the Invention
The present invention concerns a device for induction heating a metal product moving continuously in a protective atmosphere.
It also concerns an installation for continuous heat treatment of a moving product incorporating a device of the above kind.
2. Description of the Prior Art
The present invention applies more particularly to heat treatment in a protective atmosphere as carried out on hot coating lines for galvanization, aluminization, etc and continuous heat treatment lines such as continuous annealing lines for steel plate or patenting lines for steel wire.
Induction heating devices have long been used to heat moving metal products. The inductor employed generally comprises one or more turns carrying the high-frequency current and completely surrounding the moving product. This inductor creates a longitudinal flux, i.e. a flux in the direction of motion of the product, and thereby heats magnetic materials.
Another induction heating device that is known in itself includes a turn carrying the current disposed in a plane parallel to the surface of the moving product so that the transverse magnetic flux is perpendicular to the surface of the product.
However, if the moving product is not coated and is treated at a temperature at which is may oxidize, induction heating must be carried out in a protective atmosphere, such as a mixture of nitrogen and hydrogen, and the enclosure of the induction heating device must be totally airtight in order to prevent oxidation of the product.
In applications of this type of prior art induction heating device, as described in French patent 2 688 802, for example, an induction heating device placed directly within an airtight enclosure is used.
This airtight seal all around the induction means requires airtight lead-throughs for the electrical power supply connections to the inductor and for the cooling water circuits for cooling the induction means.
These water circuits inside the airtight enclosure represent a high risk to the quality of the protective atmosphere because the slightest leak of water will pollute this atmosphere and may oxidize the product.
Moreover, placing the inductor in the airtight enclosure enormously complicates maintenance and requires the provision of adequate openings and handling means for removing the entire induction device form the enclosure.
What is more, the moving product to be treated must necessarily be cut when the induction means are removed, unless an opening is provided in the induction means surrounding the moving product.
Additionally, when the induction means are disposed inside a furnace, support means must be provided that do not conduct electricity and that are resistant to very high temperatures.
Finally, any work on the induction means entails shutting down the entire installation because the air and gas seal is no longer assured.
In prior art induction heating devices like those described in patents U.S. Pat. No. 1,749,700 and GB 2 155 740, the induction means are disposed outside a gas-tight enclosure adapted to maintain a protective atmosphere around the moving metal product.
However, in these embodiments there is no electrically insulative enclosure extending beyond the induction means.
Conductive parts of the enclosure can therefore carry electrical currents induced by the induction flux returning to the induction means.
An aim of the present invention is to remedy the aforementioned drawbacks and to propose an induction heating device for a moving metal product in a protective atmosphere guaranteeing the best possible air and gas seal around the moving product, facilitating work on the induction means and limiting overheating of the device.